Process for making semi-finished products

ABSTRACT

An apparatus and a process for making semi-finished products in the form of thin metal bars having a width-gauge ratio of over 60 and a maximum sheet metal gauge tolerance of 2%. A metal profile is fed continuously and upwardly through a pool of melt material having the same composition as the metal profile so as to form a coated metal profile. The metal profile is fed at a rate which would result in a coated metal profile having a thickness of at least three times that of the uncoated metal profile. The coated metal profile is subjected to a smoothing pass between a pair of smoothing rolls when the mean temperature in the crystallized layer of the coated metal profile meets a given condition. The smoothing rolls are adjustably disposed inside a housing at a distance of 0.5 to 5 m from the melt pool surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process and a device for making semi-finishedproducts in the form of thin metal bars in which, an uncooled, cleanedmetal profile having a low heat content is run continuously from thebottom to the top through a melt pool of material of the samecomposition.

2. Description of the Prior Art

A process and a device for producing thin metal bars are disclosed in EP0 311 602 B1. A metal profile, for example, in the form of a strip-typesteel sheet (blank) having a clean surface and a thickness of 0.1 to 1.4mm, is run continuously through the bottom of a melt pool containerfilled with a steel melt of the same composition. For this purpose,there is a slot-type opening in the bottom of the melt container that isequipped with a sealing device for preventing the melt from flowing out.The temperature of the melt lies in the vicinity of the liquidustemperature T_(liq). The steel strip moves through the melt at aconstant speed and passes out of the melt upwardly. Because of the lowheat content of the steel strip (strip temperature is approximatelyequal to room temperature), an adherent layer of crystallized and stillmolten melt develops on its surface. The thickness of this layer may beseveral times the thickness of the original blank. The thickness of thelayer depends, on retention time in the melt (speed of blank), the melttemperature (temperature difference relative to the solidus temperatureT_(sol)), the melt heat and the specific heat of the material used, andthe thickness of the blank. The operation must be conducted in such amanner as to avoid remelting the already adherent crystalline likestructure. Under these conditions, a temperature gradient is inducedacross the thickness of the strip. As the strip moves through the meltpool, the temperature is lowest in the interior of the blank and risestoward the edge. A qualitatively similar temperature curve is alsopresent in the adherent layer. The temperature in the outermost regionof the layer, is the liquidus temperature, T_(liq).

Initially, the adherent layer consists of a mixture of crystalline likestructure and molten melt (mushy zone). The portion of the molten phasesin the layer increases in a direction toward the melt. After leaving themelt pool, blank and the adherent layer cool, whereby the temperaturegradient that has existed until now is reversed. The adherent layer thensolidifies completely.

From EP 0 311 602 B1 also discloses that the semi-finished productproduced as described above, after it leaves the melt pool and until itcools or enters a forming machine to undergo a hot or cold formingprocess, is to be kept in an atmosphere for protection againstoxidation. A portion of the total amount of finished product produced inthis manner is then fed back to the start of the process as blank andrun through the melt pool once again.

Until now, a crucial obstacle has hindered the practical application ofthis process in making steel strip material. Consumers of high-qualitycold or hot strip demand among other things, that the range of deviationin sheet metal thickness be no greater than 2% of the nominal thickness.A tight tolerance of this kind cannot be reliably maintained using theaforementioned process. Irregularities in strip thickness which existafter the strip left the melt pool and which exceed the prescribedmaximum limit are practically impossible to eliminate by means ofsubsequent forming procedures. This is because, given the extremeflatness of the semi-finished product used in the rolling process(width-gauge ratio of at least 60), the subsequent forming (withdecreasing thickness) takes place, essentially in the longitudinaldirection only; no further significant undoing occurs. Existingdifferences in thickness, along a line at a right angle to thelongitudinal direction of the strip, therefore remain, relativelyunchanged.

EP 0 311 602 B1 also describes another embodiment of the processwherein, in a reverse fashion, the blank is introduced into the meltpool from above and then drawn through the bottom of the melt vessel.The problem of sealing the bottom is particularly serious, for thisembodiment because the outflow directions of the melt and the stripmaterial are the same. As a result, not only is there no dynamic sealingeffect, but there is also a negative "carry along effect" which helpsinduce the melt to flow out of the vessel. For this reason, a specialsealing device in the form of a sealing roll pair is positioned in thebottom region of the melt vessel. This sealing roll pair causes adrastic compression of the "mushy zone", and thus large portions of themolten phase are squeezed out of the already formed "spongy" crystallinelike formation. Consequently, the thickness attainable in the adherentlayer, compared to the first embodiment, is considerably smaller. Foreconomic considerations alone, such a process is unsuitable forpractical applications.

SUMMARY OF THE INVENTION

An object of the invention is to further develop an apparatus and aprocess for making a thin metal bar having a maximum sheet-metal gaugetolerance of 2%.

In accordance with the present invention, a process for making asemi-finished, thin metal bar includes the steps of (a) feedingcontinuously and upwardly a metal profile through a pool of meltmaterial of the same composition as that of the metal profile so thatthe metal profile is coated with an adherent layer of melt andcrystalline structures; (b) setting a rate of feeding such that thecoated metal profile attains a thickness of at least three times athickness of the metal profile; (c) providing an inert atmosphere to aregion where the coated metal profile exits the melt pool so as toprevent the coated metal profile from oxidizing; and (d) reducing thethickness of the coated metal profile by subjecting the coated metalprofile to a smoothing pass when the adherent layer thereon attains amean temperature, T_(gl), which satisfies the following equation:

    T.sub.gl =T.sub.sol +a×(T.sub.liq -T.sub.sol)

where a is a factor having a value of 0.1 to 0.8, T_(sol) is a solidustemperature of the melt material, and T_(liq) is a liquidus temperatureof the melt material, so that the coated metal profile has a width-gaugeratio of 60 and a maximum variation in thickness of 2%.

Advantageous further developments of the invention include employing theprocess to make a thin metal bar having a thickness of less than 20 mmand applying factor a having a value in the range of 0.2 to 0.4. Stillfurther developments include reducing the thickness of the coated metalproile from 5 to 15%, selecting a rate of feeding so that a ratio of thethickness of the coated metal profile to that of the metal profile liesin a range of 3 to 7, and providing a housing for enclosing a regionwhere the coated metal profile exits from the melt pool and cooling aportion of the housing upstream the smoothing pass so as to controlcooling of the coated metal profile prior to the smoothing pass. Yetfurther advantageous developments include cooling the portion of thehousing upstream the smoothing pass to a temperature which deceleratesnatural convective cooling of the coated metal profile, cooling theportion of the housing upstream the smoothing pass to a temperaturewhich accelerates natural convective cooling of the coated metalprofile, and subjecting the coated metal profile to controlled coolingdownstream the smoothing pass.

Another object of the invention is to provide an apparatus for making asemi-finished, thin metal bar, which includes a container for containinga pool of melt material, the container having an opening in a bottomwall shaped to accommodate passing of a metal profile therethrough. Thebottom wall includes a seal disposed in the opening for continuoussealing engagement with the metal profile so as to prevent outflow ofthe melt material as the metal profile is fed therethrough. Theapparatus further includes a transport, disposed upstream the container,for feeding the metal profile through the container and a housing,disposed over the container, for enclosing a region where the metalprofile coated with a layer of melt and crystalline structures exits themelt pool. The apparatus still further includes a smoothing rollmechanism, disposed inside the housing and at a vertical distance of 0.5to 5 m from a top surface of the melt pool, for reducing the thicknessof the coated metal profile and an adjusting mechanism, operativelyconnected to the smoothing roll, for adjusting the vertical distance ofthe smoothing roll mechanism from the top surface of the melt pool. Theapparatus of the present invention is suitable in principle forproducing profiles of other types (e.g., round shapes or shapes with anydesired polygonal cross-section).

Still another object of the invention is to provide an apparatus formaking a thin metal bar of less than 20 mm in thickness. Yet anotherobject of the invention is to provide an apparatus wherein the openingof the container is shaped like a slot so as to accommodate passage of astrip-like metal profile having a width-gauge ratio of at least 60, andthe smoothing roll mechanism includes a pair of smoothing rolls spacedfrom each other. Still yet another object of the invention is to providean electromechanical or hydraulic mechanism for the adjusting mechanism.Still further object of the invention is to provide an apparatus whereinthe housing includes thermal insulation for insulating a portion of thehousing disposed proximate the smoothing roll mechanism. Yet furtherobject of the invention is to provide liquid cooling mechanism to thehousing. Still yet further object of the invention is to provide atemperature sensor, disposed proximate the smoothing roll mechanism, formeasuring a surface temperature of the coated metal profile.

BRIEF DESCRIPTION OF THE DRAWING

The invention is explained in more detail below in reference to anembodiment of the invention illustrated schematically in the drawing.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

A metal coil 12, which is unwound at a particular speed, is used as ablank. Reference number 11 denotes a strip welding unit which connectsthe end of an already unwound coil to the new coil 12, so that theprocess can be carried out continuously. Reference number 7 indicates astrip storage unit which can collect a brief stoppage of the strip feedduring the welding procedure in the event of a coil change, so that theproduction process is not interrupted. A strip cleaning device 6, whichmetically cleans the surface of the blank, is located downstream thestrip storage unit 7 in the production stream. A transport roll pair 2ensures that the blank, which has a width-gauge ratio of at least 60,preferably at least 100, is fed into the melt 3 at a constantpreselected speed through a suitable slot-type opening in the bottom ofthe melt container 1. The blank has a very low heat content, because itis at room temperature, for example. The melt 3 (e.g., steel) consistsof the same material as the blank. A seal located on the bottom of themelt container 1 is not shown separately in the drawing. As the blankpasses upwardly through the melt 3 from the bottom to the top, a layercrystallizes on the surface of the blank. The thickness of the layerincreases as the retention time increases (i.e., as the surface of themelt pool is approached), because the blank absorbs heat from itsimmediate surrounding in the melt 3. Otherwise, the melt 3 is kept at atemperature of, for example, 10 degrees K (Kelvin) above the liquidustemperature. By means of a feed not shown, the level of the melt poolsurface is kept constant. Taking into account of these and otherparameters (especially the solidus temperature, melt heat, and specificheat of the melt material), the strip speed is preferably set throughthe transport rolls 2 so that, upon leaving the melt 3, the blank andthe adherent layer attains a thickness that is three to seven times thatof the original blank.

Dispose above the melt pool surface, is a smoothing roll device in theform of a smoothing roll pair 4 positioned adjacent one another. Thedistance of this smoothing roll pair 4 from the melt pool surface can bechanged by adjusting the vertical position of the smoothing roll pair 4,for example, by means of an electromechanical or hydraulic adjustmentmechanism, which is indicated by the arrow 20 in the drawing. Theminimum distance of the smoothing roll pair 4 from the melt pool surfaceis approximately 0.5 m and the maximum distance is 5 m. The verticalposition is selected in such a way that the smoothing pass occurs at alocation where the layer adhering to the blank is relatively solidified,but nonetheless still has adequate proportions of molten phase in itsouter region, which would permit a free flow of material even at a rightangle to the longitudinal direction of the blank. What is important,therefore, is to achieve the best possible component ratio of solidphase to liquid phase. The mean temperature in the crystallized layercan be used as a control variable for this purpose. According to theinvention, smoothing is to be carried out at a temperature T_(gl) whichsatisfies the following equation:

    T.sub.gl =T.sub.sol +a×(T.sub.liq -T.sub.sol)

Where T_(gl) is the mean temperature of the crystallized layer, T_(sol)is the solidus temperature of the melt material and T_(liq) is theliquidus temperature of the melt material, and a is a factor in thevalue range of 0.1-0.8, preferably in the range 0.2-0.4. The lower thevalue of a is, the greater the solidified portion. The lower limit setsa critical threshold because, total or almost total solidification canoccur, in the layer therefore making it almost impossible to offset anylarge variations in strip thickness which might exist. The upper limitof the value a is determined primarily by economic considerations. Dueto the high proportion of molten phase, a considerable portion would besqueezed out in the downward direction due to the vertical travel of thestrip material, so that output would be correspondingly reduced. Tofacilitate adjustment, a strip surface temperature measurement device(22) can be provided in the adjustment area of the smoothing roll pair4. The smoothing roll pair 4 is advantageously provided with internalliquid cooling (e.g., water cooling). The desired reduction in metalstrip thickness during the smoothing pass should be in the range of 5 to15%.

In order to avoid oxidation of the strip surface, which interferes withthe further processing of the semi-finished product, the adherent layeron the blank can be protected against the influx of oxygen by a housing5, flooded with an inert gas. The housing 5 attaches directly to themelt container 1 and houses the smoothing roll pair 4. In order toprevent desired rapid cooling of the adherent layer and excessivelycomplete solidification this would cause, it is possible to equipportions of the walls of the housing 4 with thermal insulation asnecessary, particularly in the adjustment zone of the smoothing rolldevice 4. Apart from this, it is useful to design the walls of thehousing 5 as cooling walls, that are particularly as walls liquid-cooledfrom the inside (e.g., water cooling). By controlling the coolanttemperature, it then becomes possible to carry out controlled cooling ofthe semi-finished product in the cooling zone 8 downstream the smoothingroll device 4, so that the product attains in especially favorablematerial properties. As in the case of continuous annealing, the striplike material is run in loops in a middle section of the cooling zone 8by means of appropriate deflector rolls, so as to lengthen the time thestrip-like material is retained in this zone. After the metal stripundergoes sufficient cooling, it leaves the housing 5 having the inertatmosphere and can, for example, be oiled by an electrostatic oilingdevice 9 for protection against corrosion. The material is then woundcontinuously into a coil 13. The coil 13, after reaching a certainweight, is separated from the rest of the strip by means of a shears 10and transported away for further processing in a hot or cold rollingmill.

Of course, as was disclosed in EP 0 311 602 B1, it is also possible forthe further processing to follow immediately. In this case, it ispossible to discountinue cooling, as needed, temperature far above roomtemperature in order to save heat energy, and the housing with the inertatmosphere can be extended up to the attached forming machine.

The invention is described in greater detail in the following example,wherein reference is made to the Example;

A cold strip of an X60 steel containing

0.16% C;

0.35% Si;

1.30% Mn;

0.013% P;

0.003% S;

0.041% Al;

0.025% Nb;

0.0092% N;

Remainder: iron and common impurities.

The strip had a thickness of 0.5 mm and a width of 1000 mm and, afterbeing degreased in a pickling bath 6, it was transported verticallythrough the bottom of a melt vessel 1 filled with molten steel using thetransport roll pair 2. The melt had an analysis simulator comparable tothat of the steel strip described above. Molten steel was fedcontinuously into the melt vessel 1 from a distributor (not shown). Thelevel of the melt pool 3 and the speed of the steel strip are thecontrol variables for setting the desired contact time between the steelstrip and the melt pool 3. The contact time in the present case wasapproximately 2 sec. Because the strip speed was 1 m/s, a melt poollevel of 2 m was maintained continually. During the passage of the steelstrip through the steel melt 3, having a temperature of approximately1512° C., a crystallization layer having an overall thickness ofapproximately 2.5 mm developed, so that the total thickness of the steelstrip upon leaving the steel melt 3 was approximately 3 mm. Inaccordance with the formula T=T_(sol) +a×(T_(liq) -T_(sol)) (here a=0.5,selected), this steel strip having a "pasty" surface (two phases: meltand crystal) was then, at a mean temperature of T=1497° C.+0.5×(1507°C.-1497° C.)=1502° C. in the deposited layer, introduced into thevertically adjustable smoothing mill 4, located in a housing 5 which wascooled in a controlled fashion and filled with, for example, argon. Themaximum thickness of the steel strip was thereby reduced byapproximately 17% (0.5 mm) and its surface roughness was for the mostpart removed. In order to attain the desired objective under theexisting conditions, an integral temperature of 1502° C. proved to beespecially favorable for carrying out the smoothing pass according tothe invention. The smoothing device 4 was set in a vertical positionsuch that this temperature existed on the entrance side of the smoothingmill under the given cooling conditions. The smoothing pass which wascarried out resulted in a steel strip that was completely cavity-freeand optimally welded in its lamination and had a uniform thickness ofapproximately 2.5 mm. The deviation of the actual strip thickness fromthe target strip thickness was, only 1.6%, clearly below the maximumpermissible tolerance of 2% for hot strip which will be furtherprocessed cold. After leaving the smoothing mill 4, the steel strip, wasprotected against oxidation by an argon atmosphere, and subjected tocontrolled cooling in the water-cooled dome of the housing 4 and, afterpassing through a similarly cooled buffer area (cooling zone 8) filledwith argon, was fed to a winding station 13. After this, the steel stripwas rolled out again in a cold mill (not shown) to a thickness of 0.5mm. The cold strip produced in this manner had outstanding metallurgicaland mechanical properties and met all quality requirements.Approximately 20% of the continuously produced quantity of steel stripwas fed back to the process as starting material.

The present invention makes it possible to produce, in a surprisinglysimple manner, a strip-type metal bar which is extraordinarily accuratewith respect to its form and surface tolerance (deviation in shape andthickness is less than 2% over the length of the strip). At the sametime, this process ensures continuously reliable bonding of the adherentlayer to the blank. The option of controlled cooling permits a stripmaterial to attain excellent material properties be attained.

We claim:
 1. A process for making a semi-finished, thin metal bar havinga thickness of less than 20 mm, comprising the steps of:(a) feedingcontinuously and upwardly a metal profile through a pool of meltmaterial of the same composition as that of the metal profile so thatthe metal profile is coated with an adherent layer of melt andcrystalline structures; (b) setting a rate of feeding such that thecoated metal profile attains a thickness of at least three times athickness of the metal profile; (c) providing an inert atmosphere to aregion where the coated metal profile exits the melt pool so as toprevent the coated metal profile from oxidizing; and (d) reducing thethickness of the coated metal profile by subjecting the coated metalprofile to a smoothing pass when the adherent layer thereon attains amean temperature, T_(gl), which satisfies the following equation:

    T.sub.gl =T.sub.sol +a×(T.sub.liq -T.sub.sol)

where a is a factor having a value of 0.1 to 0.8, T_(sol) is a solidustemperature of the melt material, and T_(liq) is a liquidus temperatureof the melt material, so that the coated metal profile has a width-gaugeratio of 60 and a maximum variation in thickness of 2%.
 2. The processof claim 1, wherein said thin metal bar has a thickness of less than 20mm.
 3. The process of claim 1, wherein the factor a has a value of 0.2to 0.4.
 4. The process of claim 1, wherein step (d) the thickness of thecoated metal profile is reduced from 5 to 15%.
 5. The process of claim1, wherein step (b) the rate of feeding is selected so that a ratio ofthe thickness of the coated metal profile to that of the metal profilelies in a range of 3 to
 7. 6. The process of claim 1, prior to step (d),further comprising the steps of providing a housing for enclosing aregion where the coated metal profile exits from the melt pool andcooling a portion of the housing upstream the smoothing pass so as tocontrol cooling of the coated metal profile prior to the smoothing pass.7. The process of claim 6, wherein the portion of the housing upstreamthe smoothing pass is cooled to a temperature which decelerates naturalconvective cooling of the coated metal profile.
 8. The process of claim6, wherein the portion of the housing upstream the smoothing pass iscooled to a temperature which accelerates natural convective cooling ofthe coated metal profile.
 9. The process of claim 1, further comprisingthe step of subjecting the coated metal profile to controlled coolingdownstream the smoothing pass.